This invention relates generally to radiation imagers and in particular to large area solid state fluoroscopic imagers.
Radiation imaging commonly used for medical purposes includes radiography and fluoroscopy, a real time imaging technique. Radiography typically involves the use of higher doses of radiation to generate a low noise still image (such as on traditional x-ray film) whereas fluoroscopy typically involves the use of lower radiation levels as the total time of radiation exposure in a real time imaging procedure is longer than that required for most radiographic procedures.
Solid state devices for radiation imaging include, for example, scintillating materials that are radiation absorptive and that generate light in response to the absorption of the incident radiation, and photosensor arrays that detect the light from the scintillating materials. The electrical signals generated in the imager correspond to the intensity and spatial location of the detected incident radiation; such signals are electrically coupled to readout electrical circuits that are adapted to provide the desired presentation of images detected by the radiation imager. Solid state imaging devices are readily adapted for digital processing and are less bulky and heavy than equivalent conventional equipment, and provide performance advantages compared to the relative low dynamic range, low sensitivity, non-linear response to incident radiation, and large background fog levels inherent in most analog devices.
It is desirable that sold state radiation imaging equipment also exhibit the desirable characteristics of conventional screen or film devices, such as a large field of view, good resolution, good large-area contrast.
These desired performance characteristics of solid state fluoroscopic radiation imagers present numerous challenges with respect to the design and fabrication of switched matrix address arrays used to read respective pixels in the photosensor array. Conventional thin film transistor arrays, such as used in liquid crystal arrays, for example, typically exhibit noise levels in excess of that acceptable for a radiation imagers (transistor-generated noise generally not presenting a significant issue for satisfactory performance of a liquid crystal display). Resolving the sources of noise and identifying transistor array structures that provide low noise performance while maintaining other desired imager array operating characteristics, such as low lag, requires careful design.
It is thus an object of this invention to provide a solid state fluoroscopic radiation imager that exhibits low noise.
It is a further object of this invention to provide a solid state fluoroscopic imager that exhibits relatively low lag.
A still further object of this invention is to provide an imager that exhibits a high signal output per incident x-ray and a relatively low output capacitance.